Cooled cast turbine airfoils and other turbine section components such as blade outer air seals (BOAS) are commonly used in gas turbine engines to enable components to operate at higher gas path temperatures than would be possible with un-cooled configurations. Cast superalloys using conventional techniques such as a casting having ceramic core and/or refractory metal core cooling designs have been effective; however, demand for ever-increasing engine efficiency and reduced fuel consumption continue to challenge conventional turbine airfoil casting technology.
The ability to produce an actively cooled turbine engine airfoil that can operate thousands of hours in a thermal environment where metal temperatures operate less than 200 degrees Fahrenheit of the melting temperature of the superalloy is important. This has been achieved using a combination of cored passages and laser drilled holes and/or electrodischarge machine drilled holes that communicate with one another to provide passageways within the superalloy casting for which cooling air can enter and exit. This enables the superalloy material to retain sufficient mechanical properties to withstand operational induced loads and achieve or exceed part life requirements.
The smallest ceramic core configuration that can be produced with high production yields, (i.e., without fracture during handling, shipping and casting of product) is a shape approximately 0.050 inches (0.02 cm) to 0.025 inches (0.01 cm) in diameter. Ceramic core configurations exhibit what is known in the industry as core shift, which often occurs when the molten metal is poured onto the ceramic core. For that reason, cores can not be placed as close to the surface of the part as would be possible if there were no core. With the passageways distanced from the surface to compensate for core shift, the cooling is less effective.
Refractory metal core technology offers the ability to achieve sizes approaching 0.012 inches (0.005 cm) to 0.010 inches (0.004 cm) in one dimension, but the second or third dimensions will be greater. Refractory metal cores are generally produced from sheet stock, hence the final shape is more rectangular in cross-sectional shape than shapes achieved with ceramic core technology. Core breakage is a concern for ceramic cores, while refractory metal cores have shape limitations.
U.S. Pat. No. 4,753,575 shows airfoils with nested cooling channels where two channels carry separate coolant across the span of the airfoil in adjacent parallel paths. The paths are relatively large. Similarly, U.S. Pat. No. 5,931,638 shows blades or vanes with medial passages for coolant. The disclosures of both of these patents are incorporated herein by reference in their entirety.